A variety of printers are designed to create a printed image by printing a series of picture elements (pixels) on a print medium such as paper. Laser printers, which are a form of electrophotographic printer, create an image by scanning a laser beam across a charged surface of a photosensitive drum in a succession of raster scan lines. Each scan line is divided into pixel cells and the laser beam is modulated such that pixel cells are either exposed or not exposed depending upon the particular image being printed. Wherever a pixel cell is illuminated by the laser beam, the photosensitive drum is discharged and may then be toned in the known manner.
In laser printers, a bit map memory typically includes a digitized quantized image of a desired analog image. As such it consists of a large number of discrete pixels organized in a predetermined raster pattern. The ultimate resolution of an image produced by a laser printer is dependent upon the number of pixels printed per inch. However, even with a high pixel count level of resolution, diagonal lines and boundaries between different regions of an image appear as jagged steps or staircase distortion that is visible to the human eye.
The prior art references have considered methods for improving pixel image resolution. In U.S. Pat. No. 3,573,789 to Sharp, a 3.times.3 pixel cell subset is sequentially stepped across a bit map and, at each step, a central pixel is modified in accordance with the logic states of each of eight surrounding pixels. The central pixel may be printed as either a full pixel or as a quarter, half, or three quarter pixel.
U.S. Pat. No. 4,437,122 to Walsh et al., describes a similar nine pixel subimage but enhances the center pixel by selectively energizing one or more of nine subpixels contained within the center pixel. A table search is performed to match a prestored pixel image with an input pixel image. Among the prestored pixel images are rotated versions (90.degree., 180.degree., and 270.degree.) of the basic prestored pixel image. If a match is found, it is known that the center pixel is to be modified by turning on (or off) one or more of the nine subpixels within the center pixel.
U.S. Pat. No. 4,450,483 to Coviello performs similar pixel subimage matching as is taught by Walsh et al., however Coviello decides only whether to modify the entire center pixel, rather than substituting for it, a set of subpixels.
U.S. Pat. No. 4,847,641 to Tung, assigned to the same assignee as the application, also enhances the printing of bit-map images by matching of pixel subimages with predetermined, stored templates or patterns to detect occurrences of preselected bit map features. Whenever a match occurs, a signal is generated to produce a corrected pixel that replaces a center pixel in the pixel subimage. The stored pixel templates represent pixel configurations that are common to all bit map images and are compiled into a matching table.
Referring to FIG. 1, the logic employed by Tung to achieve an output subpixel is illustrated. An input pixel window 10 comprising a matrix of cells (with edges eroded) is accessed from a pixel bit map. Center pixel 12 is the pixel that will be altered, (if necessary), based upon a comparison of input window 10 with a plurality of templates stored in a template store 14. A circle in a pixel cell illustrates a white pixel, a black circle illustrates a black pixel, and each cell that has neither a white or black circle is a "don't care".
In the Tung system, input window 10 is fed to a template store 14 and is compared with a plurality of templates. The templates are stored in a nonrotated version, and in rotated versions at 90 degrees, 180 degrees, and 270 degrees. Associated with each stored template is a subpixel control indication which causes a laser printer to produce one of subpixels 16, 18, 20 or 22, depending upon which template is found to match input window 10. In the case shown in FIG. 1, input window 10 matches template 24 thereby resulting in output lines 26, 28, and 30 being in the OFF state and output line 32 in the ON state. The ON state of output line 32 signals a laser controller in the laser printer to print output subpixel 34. More specifically, output line 32 causes the laser printer to substitute output subpixel 34 for center pixel 12 to achieve an improved edge representation. The pixel outputs on lines 26, 28, 30 and 32 are indicated as L50 (i.e. left 50 percent of the pixel is ON), R50 (i.e. right half of the pixel is ON), T50 (i.e., top of the pixel is ON) and B50 (i.e. bottom of the pixel is ON).
In carrying out the Tung procedure, hundreds of templates are provided. Many of the templates are rotated and/or mirror image versions of fundamental templates. Thus, there is considerable redundant information in the rotated templates within the Tung memory. However, such information is required to detect all orientations of the particular input image distortion that each fundamental template was designed to detect.
Accordingly, it is an object of this invention to provide an improved pixel resolution enhancement method that employs reduced template storage.
It is another object of this invention to provide an improved resolution enhancement system for a pixel image representation which is rapid in execution and employs minimal prestored comparison templates.